The present invention relates to the field of isolators and, in particular, to isolators that operate in a wavelength dependent manner. The present invention also relates to attenuators.
In optical component systems, it is often necessary to amplify and mix optical signals. For example, one form of common amplification is to utilise a pump laser to amplify an Erbium doped fibre so as to cause an input transmission signal to be amplified to thereby form part of an output transmission signal.
Further, in amplification systems, unwanted feedback can also lead to undesirable effects. For example, a common pump amplification wavelength in the aforementioned Erbium doped fibre systems is approximately 1550 nanometres. An example wavelength for utilisation in transmission is approximately 1470 nanometres.
In any pumping system, it is desirable so as to isolate or attenuate the extent of any feedback of the pump wavelength into previous stages of the system and hence it is desirable to isolate the pump wavelength from other wavelengths which may comprise the normal operational parts of the system.
An example of a known form of wavelengths specific isolator is illustrated schematically in FIG. 1, wherein a pump laser 1 emits light at 1550 nanometres through an Erbium doped lasing section 2. A wavelength selective di-electric mirror 3 reflects wavelengths at 1550 nanometres along path 4 whilst acting in transmission at other wavelengths. The isolator 5 acts to isolate the emissions 4. Light input at the input end 6 is split by second di-electric mirror 7 which reflects light at 1550 nanometres along path 8. The light is able to pass through the isolator 5 which only isolates the return path. Wavelengths outside of 1550 nanometres pass through the dielectric mirror 7 along path 9 and through the second dielectric mirror 3 where they are again joined with the light at 1550 nanometres passing through isolator 5. They can be subsequently amplified by amplifier 2 before being output 10.
The arrangement of FIG. 1 has a number of draw backs. Firstly, it relies on a large number of components which is like to add expense to the overall system. Further, the use of the dielectric mirrors may be non ideal and hence lead to performance degradation.
Further, in optical signal processing applications, it is often desirable to attenuate a first range of frequencies in a controlled manner relative to a second range of frequencies. For example, Erbium doped fibres often exhibit frequency dependant amplification properties. The frequency dependency can also be variable with respect tot he optical power being input to the amplifier. If left unaccounted for, this can lead to undesirable effects in wavelength division multiplexed (WDM) systems. Therefore, it is generally desirable to be able to provide a frequency dependant amplitude attenuation of an optical signal.
It is an object of the present invention to provide an alternative form of wavelength dependent isolator having improved operational characteristics and a simplified form of construction.
In accordance with a first aspect of the present invention, there is provided a wavelength dependent optical device comprising: at least two optical Ports; a first series of optical components placed between the two Ports and a first dielectric mirror; a first dielectric mirror operating substantially in reflectance mode at a first predetermined wavelength range and substantially in transmission mode at a second predetermined wavelength range; at least one second optical component placed between the first dielectric mirror and a second mirror; a second mirror operating substantially in reflectance mode at the second predetermined wavelength range; wherein light emitted from a first Port at the first predetermined wavelength is transmitted to a second Port, and light emitted from the first Port at the second predetermined wavelength is transmitted away form the second Port.
Preferably, the device is also adapted such that light emitted from the second Port at the first predetermined wavelength can be transmitted to the first Port, and light emitted from the second Port at the first predetermined wavelength is transmitted away form the first Port.
The first series of optical components can comprise a first walk off plate; a first half wave plate; focussing means for focussing light emitted from the Ports; and a first non-reciprocal rotator. The second optical component can comprise a second non-reciprocal rotator.
In an alternative embodiment, the optical components can comprise a walk off plate; a series of reciprocal rotators rotating different Portions of polarised light in opposite directions; a first non-reciprocal rotator; a second walk off plate at a different orientation to the first walk off plate, a second reciprocal rotator; a third reciprocal rotator; and focussing means for focussing light emitted from the optical Ports.
Preferably, a first one of the predetermined wavelength ranges corresponds to the pump wavelength range of an optical fibre laser device and a second one of the predetermined wavelength ranges correspond to a signal transmission wavelength through an optical fibre communications system.
In accordance with a further aspect of the present invention, there is provided in a reflective isolator device, a method of providing a wavelength dependant isolator comprising the steps of: transmitting light emitted through a first Port through a series of polarisation modifying optical components; utilising a dielectric mirror to filter out a first wavelength range from other wavelength ranges of the transmitted light; manipulating the polarisation state of the first wavelength range; recombining the manipulated first wavelength with the other wavelength ranges of the transmitted light; transmitting the recombined light through a second series of polarisation modifying optical components towards a second Port; such that light emitted from the first Port in the other wavelength ranges is directed towards the second Port whereas light emitted from the first Port in the first wavelength range is transmitted away from the second Port.
Further, preferably light emitted from the second Port in the other wavelength ranges is directed towards the first Port whereas light emitted from the second Port in the first wavelength range is transmitted away from the first Port.